1. Field of the Invention
The present invention relates to an optical scanning lens, an optical scanning device and an image forming apparatus.
2. Description of the Related Art
In an optical scanning device widely known with reference to a laser printer, a digital copier, and so forth, a beam emitted from a light source is deflected by a light deflector, the deflected beam is condensed toward a surface to be scanned by a scanning and imaging optical system, a beam spot is formed thereby on the surface to be scanned, and thus, optical scanning of the surface to be scanned is performed.
Recently, a plastic-made (made of plastic) optical scanning lens is employed as the scanning and imaging optical system or as a part thereof.
The plastic-made optical scanning lens is formed through plastic mold. As this type of lens can be easily mass-produced, it can be manufactured at low costs. Accordingly, by employing the plastic-made optical scanning lens, it is possible to effectively reduce the costs of the scanning and imaging optical system, and, thereby, the costs of the optical scanning device. Further, with regard to the plastic-made optical scanning lens, it is possible to easily form a special lens surface shape such as an aspherical surface through plastic mold. Accordingly, it is possible to simplify the scanning and imaging optical system (reducing the number of lenses required) and/or to improve optical performance thereof.
However, this type of plastic-made optical scanning lens has a problem in that a non-uniform distribution of the refractive indexes occurs inside of the plastic-made optical scanning lens.
Through plastic mold, molding is performed in which thermally molten plastic material is injected into a die, and is cooled in the die. At this time, cooling begins from a part of the material in contact with the die. Accordingly, the center of the plastic material is cooled slowly relative to the periphery thereof. Thereby, a non-uniform distribution of density (the density at a part cooled rapidly becomes higher than that at a part cooled slowly) inside of the plastic. As a result, the distribution of the refractive indexes of the thus-formed lens is not uniform inside thereof. As the density of the periphery of the formed lens is higher than that of the center thereof, the refractive index is lower at the lens center while, the nearer to the surface the position thereof becomes, the higher the refractive index of the formed lens becomes, in general.
When the distribution of the refractive indexes of the plastic lens is measured by a method described later, the variation in refractive index is like an approximately quadratic curve along each of a lens optical axis direction, a main scanning direction and a sub-scanning direction.
The plastic-made optical scanning lens is designed assuming that the distribution of refractive indexes inside thereof is uniform. Accordingly, when the plastic lens has a non-uniform distribution of refractive indexes inside thereof, it cannot exhibit the performance according to the design. Specifically, defocus occurs such that a position of imaging of the deflected beam differs from the surface to be scanned, thereby, a position of beam waist of the deflected beam is changed from the surface to be scanned, and, as a result, the diameter of the beam spot increases.
In order to reduce such a non-uniform distribution of refractive indexes inside of the lens, it can be considered to cool the molten plastic material in the die, very slowly for a long time (for example, for ten and some hours) in a thermostatic chamber. However, by such a method, the productivity of the optical scanning lens becomes worse, and the manufacturing costs thereof increase. Accordingly, the advantage of the plastic-made lens such as requiring low costs may be cancelled.
An object of the present invention is to provide an optical scanning lens which has a no problem in optical performance/characteristics thereof, even having relatively a large distribution of refractive indexes inside thereof.
An optical scanning lens according to the present invention is an optical scanning lens used in a scanning and imaging optical system for converging a beam deflected by a light deflector onto or in the vicinity of a surface to be scanned, and is formed through plastic mold, and, a distribution of refractive indexes xcex94n(x) inside of the lens has a local maximum, within a range through which the beam passes through the lens.
The above-mentioned scanning and imaging optical system may include only a single lens, or a plurality of lenses, or at least one lens and a mirror surface (concave surface/convex surface) having an imaging function.
The optical scanning lens according to the present invention is used as at least a component of the scanning and imaging optical system, and, one or a plurality thereof is/are disposed in the scanning and imaging optical system. It is also possible that the scanning and imaging optical system is formed by the optical scanning lens itself.
As a plastic material of the optical scanning lens, any one of acrylic resin (PMMA/alicyclic acrylic resin), PC (polycarbonate), polyolefin resin (ordinary polyolefin/alicyclic polyolefin), and so forth can be used.
When a lens is formed through plastic mold by using such resin material, acrylic resin has an advantage such that the optical elastic constant thereof is small and double refraction is not likely to occur therein. PC has an advantage such that the refractive index thereof is high, also, the moisture absorbing rate thereof is small, and, the optical characteristics of the lens is not likely to be affected by the environment. Polyolefin resin has an advantage such that the moisture absorbing rate thereof is small, and double refraction is not likely to occur therein.
Any of the above-mentioned materials causes a non-uniform distribution of refractive indexes inside the lens during a process of the plastic mold therefor. Thereamong, polyolefin resin is one which causes a non-uniform distribution of refractive indexes during the process of the plastic mold, most remarkably. Therefore, the present invention is effective in a case where the polyolefin resin is used as a material of the optical scanning lens.
The above-mentioned range through which the beam passes through the lens is a range for which the beam deflected by the light deflector passes through the lens during the deflection thereof. Specifically, with respect to the main scanning direction, this range is a range through which the deflected beam passes through the lens so that the beam thus having passed through the lens scans an effective writing range on the surface to be scanned. With respect to the sub-scanning direction, the range through which the beam passes through the lens is a range determined in consideration of a possible variation in angle of emission of light from the light source, a possible inclination of the deflection reflective surface of the light deflector, and so forth. The range through which the beam passes through the lens with respect to the sub-scanning direction may be preferably xc2x12 mm from a plane parallel to the main scanning direction and including the optical axis, normally, in the scanning and imaging optical system of a laser printer or the like. The size of the range through which the beam passes through the lens may vary according to optical requirements such as the effective writing range, diameter of beam spot, and so forth.
The definition of the above-mentioned distribution of the refractive indexes xcex94n(x) inside of the lens will be described later.
The above-mentioned feature of the present invention in that the distribution of the refractive indexes xcex94n(x) has a local maximum means that, when this distribution is approximated by using a polynominal, of Ian order equal to or more than third order (practically, even order, not less than fourth order and not more than tenth order), of a variable x, within the range through which the beam passes through the lens, the thus-approximated distribution has a range in which dn/dx=0 and also d2n/dx2 less than 0, hereinafter.
It is preferable that the above-mentioned distribution of the refractive indexes xcex94n(x) also has a local minimum. This means that the above-mentioned approximated distribution also has a range in which dn/dx=0 and also d2n/dx2 greater than 0, hereinafter.
Further, it is preferable that the above-mentioned distribution of the refractive indexes xcex94n(x) satisfies the following requirement:
0.1xc3x9710xe2x88x925 less than LMAX[xcex94n(x)]xe2x88x92min[xcex94n(x)] less than 4xc3x9710xe2x88x925xe2x80x83xe2x80x83(1)
where LMAX[xcex94n(x)] denotes the above-mentioned local maximum, and min[xcex94n(x)] denotes the minimum value of the above-mentioned distribution of the refractive indexes xcex94n(x).
Further, it is preferable that the above-mentioned distribution of the refractive indexes xcex94n(x) satisfies the following requirement:
1xe2x89xa6{max[xcex94n(x)]xe2x88x92min[xcex94n(x)]}/{LMAX[xcex94n(x)]xe2x88x92min[xcex94n(x)} less than 15xe2x80x83xe2x80x83(2)
where max[xcex94n(x)] denotes the maximum value of the above-mentioned distribution of the refractive indexes xcex94n(x).
Furthermore, it is preferable that the above-mentioned distribution of the refractive indexes xcex94n(x) is a distribution in a sub-scanning section on or in the vicinity of the center of the lens in the main scanning direction.
The above-mentioned sub-scanning section means an imaginary planar section of the optical scanning lens perpendicular to the main scanning direction. Similarly, a planar section parallel to the main scanning direction and including the optical axis is called a xe2x80x98main scanning sectionxe2x80x99.
It is also preferable that the above-mentioned distribution of the refractive indexes xcex94n(x) satisfies the following requirements same as the above-mentioned requirements (1) and (2):
0.1xc3x9710xe2x88x925 less than LMX[xcex94n(x)]xe2x88x92min[xcex94n(x)] less than 4xc3x9710xe2x88x925xe2x80x83xe2x80x83(1)
1xe2x89xa6{max[xcex94n(x)]xe2x88x92min[xcex94n(x)]}/{LMAXxcex94n(x)]xe2x88x92min[xcex94n(x)]} less than 15xe2x80x83xe2x80x83(1)
The optical scanning device according to the present invention deflects the beam coming from the light source at a uniform angular velocity by the light deflector having the deflection reflective surface, condenses the deflected beam on the surface to be scanned as a beam spot by the scanning and imaging optical system, and, thus, performs optical scanning of the surface to be scanned at a uniform velocity.
As the light source, any one of various types of solid lasers, gas lasers, LEDs, and so forth may be used. However, a semiconductor laser is most preferable. The above-mentioned surface to be scanned is substantially, a photosensitive surface of a photosensitive medium (for example, a photoconductive photosensitive body).
In the above-mentioned optical scanning device, the above-mentioned optical scanning lens according to the present invention is used at least as a part of the scanning and imaging optical system.
An image forming apparatus according to the present invention performs optical scanning of the surface to be scanned so as to form a latent image thereon, and visualizes the latent image so as to obtain a printed image. In the image forming apparatus, as the optical scanning device for performing the optical scanning of the photosensitive surface of the photosensitive medium, the above-mentioned optical scanning device according to the present invention is used.
The photosensitive medium may comprise a photoconductive photosensitive body, and the electrostatic latent image formed on the photosensitive surface through uniform changing and optical scanning of the photosensitive surface may be visualized into a toner image. The toner image is fixed onto a sheet-like recording medium (such as a transfer paper sheet, an OHP sheet (plastic sheet used for an overhead projector), or the like). In such a case, the image forming apparatus is embodied as a laser printer, a laser plotter, a digital copier, a facsimile machine, or the like
However, as the above-mentioned photosensitive medium, for example, a sliver bromide photographic film may be used. In this case, the latent image formed through optical scanning by the optical scanning device can be visualized through an ordinary silver bromide photographic process. In such a case, the image forming apparatus is embodied as an optical plate making apparatus, an optical drawing apparatus or the like.
Thus, according to the present invention, the optical scanning lens can have optical characteristics, occurs practically no problems even having a remarkably non-uniform distribution of refractive indexes inside thereof. Accordingly, by using the image forming apparatus including the optical scanning device employing this optical scanning lens, it is possible to render a satisfactory image formation.
Other objects and further features of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.